Does HttpSession.invalidate function kill all the sessions?

I have 2 webserver, each one tied with an app server. The session timeout has been set to 4 hours. But sometimes after couple of minutes, the user is kicked back to the login page. I am not sure if HttpSession.invalidate is the cause? Pls advise.

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>>Is that any random name, or does it have to be the userid
Possible, above is just an example on how to kill a particular session object. "test" will be the object name, while "124" is the value associated within the session object.